Light olefins such as ethylene, propylene and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, light olefins are produced by cracking petroleum feeds. Because of the limited supply of competitive petroleum feeds, the opportunities to produce low cost light olefins from petroleum feeds are limited. Efforts to develop light olefin production technologies based on alternative feeds have increased.
Oxygenates, such as, for example, alcohols, particularly methanol, ethanol, n-propanol, and iso-propanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate are an important type of alternate feed for the production of light olefins. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. For example, methanol, the preferred alcohol for light olefin production, may be converted to primarily ethylene and propylene in the presence of a molecular sieve catalyst. Oxygenate-containing feeds, particularly inexpensive oxygenate-containing feeds, may contain impurities which are deleterious to the catalysts employed in oxygenate to olefin conversion processes. Such impurities include low-volatile materials and non-volatile materials that have negligible vapor pressure at the conditions necessary to prepare feed for the oxygenates to olefin conversion process.
Although low-volatile and non-volatile materials are not normally found in freshly produced oxygenated hydrocarbons, they can be introduced during storage and handling of oxygenates, as well as during recycling of oxygenate streams to a reactor. Because of catalyst sensitivity in an oxygenates to olefins conversion reactor, even small amounts of non-volatile and/or low-volatile contaminants such as metals, salts and heavy hydrocarbons in the feed can accumulate on and thus poison the reactor's catalyst. These poisons interfere with the catalyst's function, reducing the life and selectivity of the catalyst, which results in increased overall production costs. Given that non-volatiles in the feed at levels as low as one wppm can accumulate to 12,000 wppm on the catalyst inventory, a compelling interest exists to provide feeds of extremely reduced non-volatiles content. Other processes have been taught that attempt to reduce the amount of catalyst poisons in the OTO reactor feed.
U.S. patent application Ser. No. 10/020,732, filed Oct. 30, 2001, entitled “Heat Recovery in an Olefin Production Process,” the entirety of which is incorporated herein by reference, discloses a process for removing heat from an effluent stream while maintaining a temperature of the gas phase above the dew point. By following this process, some solid particles and some other contaminants may be separated from the oxygenates. By removing these contaminants, the catalysts in the OTO reactor will perform better and last longer, thereby improving the reactor's efficiency and reducing production costs. Unfortunately, this process does not remove all catalyst poisons and further creates difficulties in maintaining the effluent at a proper temperature and pressure.
It should thus be appreciated that a delicate balance exists between providing an inexpensive contaminated oxygenate-containing feed for an OTO reactor while ensuring that the contaminants in the oxygenate-containing feed do not significantly poison an OTO reactor catalyst. That is, it is desirable to maximize the amount of contaminants in a feed stream in order to obtain low oxygenate purification costs, while maintaining a low enough level of poisoning contaminants that OTO reaction efficiency is not significantly reduced. Accordingly, it would be desirable to provide a process that effects substantial removal of poisoning contaminants, while at the same time allowing non-poisoning contaminants to enter the OTO reaction system.